During Desert Storm Teledyne Corp developed very sophisticated means to measure for chemical agents on the battlefield.The technology was soon applied to drug testing.Your hair is measured,and contains any history of drug use.Shave your head,and fingernail shavings will work.
That works both ways,I suppose.A hair test could prove you are a non user,but then,if you were a weekend user for a year,then went a month with no pot,you would probably pass a blood or urine test,but your hair could prove you a user.